I am having this same problem, quite scary as i dont have rac membership. Erm i take it leads and plugs and dizzy cap are ok? otherwise, think the hal sender can do stuff like this, let me know if you get it sorted, hope this helps. 
What is the Hal sender and where is it located?
This happened to me not so long back. Turned out to be a loose circlip on a fuel line which starved the engine of fuel, as it was leaking out a little. It would judder in the middle revs, flooring it or throuttle off and it would stop. I could smell petrol after I had stopped, so had a look and found a loose pipe. Nipped up the circlip and the juddering stopped.
Maybe worth a look. 
cheers for sugestion but theres no smell of petrol anywhere, could faulty fuel pump cause this.
Had an old guy at work (I work in IT dept

) suggesting 'bunged up injectors' and to get some cleaner for it. Really has me stumped, dont know which way to turn. Dont want to waste my money on parts I dont need. aaaahhh!!! Need to get it fixed soon though as GF is already suggesting getting
a proper car

cheers for suggestions